linux-kernel - Re: [PATCH v1 15/26] x86/sev: Introduce snp leaked pages list

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Date: Tue, 9 Jan 2024 16:19:02 -0600
From: "Kalra, Ashish" <ashish.kalra@....com>
To: Vlastimil Babka <vbabka@...e.cz>, Michael Roth <michael.roth@....com>,
 x86@...nel.org
Cc: kvm@...r.kernel.org, linux-coco@...ts.linux.dev, linux-mm@...ck.org,
 linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
 tglx@...utronix.de, mingo@...hat.com, jroedel@...e.de,
 thomas.lendacky@....com, hpa@...or.com, ardb@...nel.org,
 pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
 jmattson@...gle.com, luto@...nel.org, dave.hansen@...ux.intel.com,
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 srinivas.pandruvada@...ux.intel.com, rientjes@...gle.com, tobin@....com,
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 jarkko@...nel.org, nikunj.dadhania@....com, pankaj.gupta@....com,
 liam.merwick@...cle.com, zhi.a.wang@...el.com
Subject: Re: [PATCH v1 15/26] x86/sev: Introduce snp leaked pages list

Hello Vlastimil,

On 1/8/2024 4:45 AM, Vlastimil Babka wrote:
> On 12/30/23 17:19, Michael Roth wrote:
>> From: Ashish Kalra <ashish.kalra@....com>
>>
>> Pages are unsafe to be released back to the page-allocator, if they
>> have been transitioned to firmware/guest state and can't be reclaimed
>> or transitioned back to hypervisor/shared state. In this case add
>> them to an internal leaked pages list to ensure that they are not freed
>> or touched/accessed to cause fatal page faults.
>>
>> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
>> [mdr: relocate to arch/x86/virt/svm/sev.c]
>> Signed-off-by: Michael Roth <michael.roth@....com>
> Hi, sorry I didn't respond in time to the last mail discussing previous
> version in
> https://lore.kernel.org/all/8c1fd8da-912a-a9ce-9547-107ba8a450fc@amd.com/
> due to upcoming holidays.
>
> I would rather avoid the approach of allocating container objects:
> - it's allocating memory when effectively losing memory, a dangerous thing
> - are all the callers and their context ok with GFP_KERNEL?
> - GFP_KERNEL_ACCOUNT seems wrong, why would we be charging this to the
> current process, it's probably not its fault the pages are leaked? Also the
> charging can fail?
> - given the benefit of having leaked pages on a list is basically just
> debugging (i.e. crash dump or drgn inspection) this seems too heavy
>
> I think it would be better and sufficient to use page->lru for order-0 and
> head pages, and simply skip tail pages (possibly with adjusted warning
> message for that case).
>
> Vlastimil
>
> <snip

Considering the above thoughts, this is updated version of 
snp_leak_pages(), looking forward to any review comments/feedback you 
have on the same:

void snp_leak_pages(u64 pfn, unsigned int npages)
{
         struct page *page = pfn_to_page(pfn);

         pr_debug("%s: leaking PFN range 0x%llx-0x%llx\n", __func__, 
pfn, pfn + npages);

         spin_lock(&snp_leaked_pages_list_lock);
         while (npages--) {
                 /*
                  * Reuse the page's buddy list for chaining into the leaked
                  * pages list. This page should not be on a free list 
currently
                  * and is also unsafe to be added to a free list.
                  */
                 if ((likely(!PageCompound(page))) || (PageCompound(page) &&
                     !PageTail(page) && compound_head(page) == page))
                         /*
                          * Skip inserting tail pages of compound page as
                          * page->buddy_list of tail pages is not usable.
                          */
                         list_add_tail(&page->buddy_list, 
&snp_leaked_pages_list);
                 sev_dump_rmpentry(pfn);
                 snp_nr_leaked_pages++;
                 pfn++;
                 page++;
         }
         spin_unlock(&snp_leaked_pages_list_lock);
}

Thanks, Ashish